As is known, high temperature nuclear reactors utilize helium in a primary cooling circuit in order to cool the reactors. However, the medium circulating in these primary cooling circuits generally contain small quantities of impurities, for example, water and/or hydrogen, which react with the graphite of the reactor to form carbon monoxide and methane. Traces of nitrogen may also be present as further impurities. In addition, the ordinarily-used nickel-base alloys for the structural parts of this primary circuit contain, as alloying additives, titanium and chromium. Both of these, especially in the case of the high temperatures of some 1000.degree. C. prevailing in these circuits, react with the carbon of the carbon-containing impurities, and, through carbonizing and forming carbide, lead to a deterioration of the mechanical characteristics of the nickel-base alloys.
Accordingly, it is an object of the invention to provide a protective layer for structural components used in high temperature nuclear reactors which is able to prevent carbonization of the base material.
It is another object of the invention to form a protective layer on a nickel-base alloy containing a carbode former in a relatively simple manner.
Briefly, the invention provides a protectively coated structural component for a helium cooling circuit of a high temperature nuclear reactor as well as a process for forming the protective layer in a pore-free manner.
The structural component comprises a substratum of a nickel-base alloy containing a carbide-former, an intermediate layer of nickel of at least ninety-nine percent (99%) purity anchored on at least a portion of the substratum, and a pore-free protective layer on the intermediate layer of a metal selected from the group consisting of tungsten and molybdenum.
The protective layer is of a thickness of from 0.05 to 0.2 millimeters while the intermediate layer is of a thickness of 0.01 to 0.05 millimeters.
The process of forming the protective layer includes the steps of cleaning the substratum, galvanically depositing a layer of nickel of at least ninety-nine percent (99%) purity on the substratum to form an intermediate layer, and thereafter, a layer of a metal selected from a group consisting of tungsten and molybdenum is deposited by a gas or vapor deposition process on the intermediate layer.
The protective layer effects a bonding of the carbon in the carbon-containing impurities in the cooling circuit into the protective layer as tungsten carbide or molybdenum carbide (W.sub.2 C and Mo.sub.2 C respectively) to form a very stable layer of protection against further carbonization. Thus, the intermediate layer of pure nickel has a double function. In the first place, the intermediate layer improves the adhesion of the protective layer on the substratum. In the second place, the intermediate layer inhibits a decarbonization of the substratum through a reaction of its contained carbon with the protective layer.
In addition, the protectively coated structural components have the further substantial advantage that protection is simultaneously obtained against an oxidation attack on the basic material, while an oxidation of the protective layer itself is not to be feared, even at the maximum prevailing temperatures.
The vapor deposition process is followed by a diffusion heat treatment which may be carried out both in a vacuum of some 10.sup.-4 millimeters of mercury (mm Hg) or else in an inert-gas atmosphere, e.g. argon or helium, in known manner in order to improve the adhesion of the protective layer on the intermediate layer, or of that layer on the substratum.
Further, advantages have been found to result when the pure-nickel intermediate layer is applied through a galvanic process, e.g. electroplating, because such nickel layers are free of phosphorus or boron impurities, which might lead to undesirable reactions between the nickel and protective-layer elements.
The heat treatment is accomplished at a temperature between 800.degree. C. and 1200.degree. C., being held for about 12 hours at 800.degree. C. and about two hours at 1200.degree. C. A following cooling-down is carried out in the treatment furnace slowly, likewise in a vacuum or in an inert-gas atmosphere.
The nickel-base alloys which contain a carbide former and which are used in making the structural components for a helium cooling circuit of a high temperature nuclear reactor generally have at least the following components in percent by weight:
0.02 to 0.15% carbon PA0 5.5 to 16% chromium PA0 3 to 13% of the sum of molybdenum and tungsten PA0 6 to 11% of the sum of aluminum and titanium and base nickel i.e. the remainder is nickel with alloy components as commonly used in nickel superalloys.